I would assume there isn't a magic number which can't be exceeded, but something which can be observed usually happens around that time, i.e. the glass becomes

I would assume there isn't a magic number which can't be exceeded, but something which can be observed usually happens around that time, i.e. the glass becomes visibly cloudy and it's undesirable to sell new product in them, as opposed to invisible cracks in carbon fiber composites which require ultrasound to detect.